Intramolecular dihydrobenzofurans

The intramolecular G-H insertion of aryldiazoacetates is a very powerful strategy for synthesizing heterocyclic rings such as dihydrobenzofurans and dihydrobenzopyrans. The first effective asymmetric copper-catalyzed C-H activation reaction was reported by the group of Sulikowski in the synthesis of the anti-tumor antibiotic FR-66979 93 (Scheme 7)123 219 The G-H insertion of the aryldiazoacetate 90 formed four diastereomeric products 91a, 91b, 92a,... 

A very impressive application of this chemistry is the total synthesis of (—)-ephedradine A 102.222 The key intermediate /rcarboxylic acid ester 101 was synthesized by intramolecular C-H insertion reaction. Upon treatment with a catalytic amount of Rh2(Y-DOSP)4, aryl diazo ester 100 possessing a chiral auxiliary underwent a C-H insertion reaction to give 101 in 63% yield and 86% de (Equation (83)). 

A new synthetic method of benzofuran was reported (equation 39). The [3,31-sigma-tropic rearrangement of Af-trifluoroacetyl enehydroxylamines 136 obtained in situ by acylation of oxime ethers 135 in the presence of trifluoroacetic anhydride lead to the synthesis of cyclic or acyclic dihydrobenzofurans 138. The effects of base and temperature on the reaction products were studied. A similar pathway to that of Fisher indolization was proposed. The acylimine formed by the [3,3]-sigmatropic rearrangement of the V-trifluoroacetyl enehydroxylamine 136 gave the dihydrobenzofuran 137 by an intramolecular cyclization or the benzofuran 138 after elimination. 

The proposed mechanism is given in Scheme 15. Initially the dissociation of water, maybe trapped by the molecular sieve, initiates the catalytic cycle. The substrate binds to the palladium followed by intramolecular deprotonation of the alcohol. The alkoxide then reacts by /i-hydride elimination and sets the carbonyl product free. Reductive elimination of HOAc from the hydride species followed by reoxidation of the intermediate with dioxygen reforms the catalytically active species. The structure of 13 could be confirmed by a solid-state structure [90]. A similar system was used in the cyclization reaction of suitable phenols to dihydrobenzofuranes [92]. The mechanism of the aerobic alcohol oxidation with palladium catalyst systems was also studied theoretically [93-96]. 

Benzofurans and dihydrobenzofurans have been prepared on polymeric supports by the palladium-mediated reaction of 2-iodophenols with dienes or alkynes (Entries 1 and 2, Table 15.9). This reaction is closely related to the synthesis of indoles from 2-iodoanilines, and probably proceeds via an intermediate palladacycle (Figure 15.3). Benzofuran and isobenzofuran derivatives have also been prepared on cross-linked polystyrene by intramolecular addition of aryl radicals to C=C double bonds and by intramolecular Heck reaction. 

Intramolecular substitution for chloride or fluoride is particularly effective. Oxygen heterocycles with fused benzo rings are obtained from Cr(CO>3 complexes of fluorobenzene with an o-(hydroxyalkyl) side chain.39,60 For example, complexation of 3-(2-fluorophenyl)-l -propanol with [Cr(CO>3(pyridine)3] at 25 C in ether (promoted by BF3 Et20) followed by reaction in DMSO with excess potassium f-butoxide for 3 h at 25 C gave the chroman complex (14).60 The yield in the cyclization step was 75% and iodine decomplexation was quantitative (equation 12). Efforts to produce the dihydrobenzofuran under the same... 

The oxidative ring opening of 3-oxabicyclo[4.1.0]hept-4-enes, formed by the intramolecular Pt(ll)-catalyzed cyclopropanation of enol ethers by alkynes, gives oxepane derivatives. Alternatively, the acid-catalyzed opening of the cyclopropane ring leads to dihydrobenzofurans or 3,4-dihydro-2//-chrorncncs <20040L3191>. 

An effective and general method for the synthesis of 2,3-dihydrobenzofuran derivatives has been developed via an intramolecular carbolithiation reaction of o-lithioaryl ethers (Scheme 16).89 This process was the first example in which the carbolithiation reaction has been stopped at the 2,3-dihydrobenzofuran stage by appropriate selection of the ether moiety. 

Ring closure reactions taking place by intramolecular addition of an aromatic radical to a double bond have been widely studied on both their regio- and stereochemical aspects [93]. Aryl halides and diazonium salts substituted at ortho- position with a 0-allyl or TV-allyl chain were used for the preparation of 2,3-dihydrobenzofuranes and 2,3-dihydro-1/7-indoles under different reaction conditions. The reaction pattern involves the generation of an aryl radical 20, which reacts with the double bond in a 5-exo trig fashion to afford the exocyclic radical 21, plausible of reduction by a hydrogen donor to obtain the reduced-cyclized product 22 (Sch. 23) [93d,94]. 

Several oxidations of this kind were performed inter- and intramolecularly. In the first case with 4-methoxyphenols and electron-rich styrenes or propenylbenzenes, dihydrobenzofuran derivatives were formed [19] ... 

Another intramolecular substitution has been used for the synthesis of an intermediate in a proposed route to 3-substituted benzofurans. A / -methylene-dihydrobenzofuran complex was obtained upon fluoride-induced removal of the SiR3 protecting group from complex 37 (Scheme 16) in an ipso SnAr process. Desilylation resulted in spontaneous cyclization to the stable methylene complex 38 in 89 % yield. No isomerization occurred and the 3-methyl benzofuran complex was not detected [21]. 

Although the synthesis of dihydrobenzofuran derivatives seems to be not possible by this anionic cyclization methodology, there are some particular examples in which these heterocycles are prepared by an intramolecular carbolithiation reaction. In this respect Baldwin and coworkers described in 1980 the preparation of 350 by rearrangement of 349 when it was treated with BuLi in THF/TMEDA (Scheme 91)151. The most likely explanation starts with an orf/zo-lithiation giving a dilithium intermediate, which undergoes an intramolecular 5-exo carbolithiation reaction affording a 3-lithiomethyldihydrobenzofuran... 

An iridium(lll)-catalyzed tandem Claisen rearrangement-intramolecular hydroaryloxylation protocol has been described that allows the transformation of allyl aryl ethers to dihydrobenzofurans under mild conditions (Scheme 95) <2005TL1237>. An in rl/ -generated PPh3AuOTf complex also proved to be an efficient catalyst for this transformation <2006SL1278>. 

Intramolecular Heck reactions for building up complex oxacyclic skeletons are a common theme in the synthesis of natural products. These reactions are exceptionally valuable for the installation of quaternary carbon stereocenters. In the morphine total syntheses by Overman <1994PAC1423> and Trost et al., intramolecular Heck reactions to form dihydrobenzofurans served as strategic key steps (Equation 138) <2005JA14785>. Asymmetric variants of intramolecular Heck reactions based on BINAP ligands to yield dihydrobenzofurans have also been investigated <1998T4579>. 

When the Heck cyclization is carried out under oxidative conditions (intramolecular Fujiwara-Moritani arylation), spirocyclic dihydrobenzofurans are formed in good yields directly from aryl ethers (Equation 139) <2004AGE6144>. 

An enantioselective method for the synthesis of 3-functionalized 2,3-dihydrobenzofuran derivatives via an intramolecular carbolithiation reaction of allyl 2-lithioaryl ethers uses (—)-sparteine as a chiral inductor. A variety of electrophiles can be reacted with the cyclized organolithium intermediate. With certain substrates, however, /3-elimination occurs instead (Equation 140) <2005CEJ5397>. 

More recently, elegant mechanistic studies on intramolecular Meer-wein reactions by Beckwith et al. considerably extended the utility of diazonium salts. They showed that many electron donors could convert an arenediazonium cation into an aryl radical which cyclized in good yield to form dihydrobenzofurans and indolines. The final radical was functionalized as a halide, sulfide, or ferrocene. Thus, the credentials of diazonium salts as electron acceptors were well established, and the stage was set to investigate the interaction between diazonium salts and TTF. 

Sml2-H20-amine mediated diastereoselective intramolecular couplings were reported for the synthesis of dihydrobenzofuran, and a radical mechanism was proposed to account for this reaction <03OBC2423>. A similar approach for the synthesis of pyridine-fused polycyclic amines was also developed by use of AlBN/BUjSnH as a reducing system <03TL2995>. 

When steric and electronic features of the radical intermediates do not allow the desired radical propagation steps to be sustained, the product yield may be low. In such cases, more favorable - but often undesirable - processes, such as H-abstrac-tion from the solvent, may become dominant [503]. Especially intriguing are scaffolds on which radical and ionic cyclizations lead to different products. Thus, Ber-teina et al. described the formation of l-alkylidene-5H-dihydrobenzofuran and l-alkyl-5-dihydrobenzofuran from o-iodobenzyl vinyl ethers. The former product was formed by an intramolecular Heck reaction, while the latter was formed in high yields in a radical cyclization [504],... 

Scheme 11 Asymmetric synthesis of dihydrobenzofurans through intramolecular C-H insertion...

In 2003, Stoltz at CalTech described a palladium-catalyzed oxidative Wacker cyclization of o-allylphenols such as 55 in nonpolar organic solvents with molecular oxygen to afford dihydrobenzofurans such as 56.44 Interestingly, when (-)-sparteine was used in place of pyridine, dihydrobenzofuran 56 was produced asymmetrically. The ee reached 90% when Ca(OH)2 was added as an additive. Stoltz considered it a stepping stone to asymmetric aerobic cyclizations. In 2004, Mufiiz carried out aerobic, intramolecular Wacker-type cyclization reactions similar to 55—>56 using palladium-carbene catalysts.45 Hiyashi et al. investigated the stereochemistry at the oxypalladation step in the Wacker-type oxidative cyclization of an o-allylphenol. Like o-allylphenol, o-allylbenzoic acid 57 underwent the Wacker-type oxidative cyclization to afford lactone 58.47... 

Organocopper reagents can be made directly from the bromides and active Cu, which is prepared by reduction of CuCN nLiX with lithium naphthalenide. Copper prepared from Cul PR, (R=Bu, Ph) is most suitable for intramolecular reaction of bromoaryl epoxides to give dihydrobenzofurans. 

Cyclohexadienones of the type IV (n = 0, 1, 2)29 are formed from 2,2 -bi-phenyldiols, 2,2 -methylenebisphenols or 2,2 -ethylenebisphenols via intramolecular C-0 coupling. 5, 7, 3 , 5 -Tetra-tert-butylspiro [2,3-dihydrobenzofuran-2,P-cyclohexa-3 , 5 -diene-2 -one] (IV, R1 = R2 = tert-Bu, n = 1) is stable. The similar compound IV (R1 = tert-Bu, R2 = Me, n = 1), prepared from the technically important antioxidant 2,2 -methylenebis (4-methyl-6-tert-butylphenol) by oxidation with sodium hypochlorite30 is oily and crystallizes in the form of Diels-Alder addition compound V. 

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